Intrinsic factor is a protein produced by cells in the stomach lining. It is needed for the intestines to absorb vitamin B12 efficiently.

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Immune system damage to the stomach lining can cause a lack (deficiency) of intrinsic factor. This can lead to vitamin B12 deficiency and low red blood cell count (anemia), which is called pernicious anemia . Intrinsic factor deficiencies can be caused by other reasons, such as surgical removal of the stomach, inherited conditions, and other diseases.